Tread for pneumatic tire and tire comprising such a tread

ABSTRACT

A pneumatic tire having a block design suited for off-road use modified to improve rolling resistance and noise properties when used on paved surfaces has a tread design having angled lateral groove between the blocks in each circumferential row of blocks in the tread. Narrow grooves separate the blocks in several of the rows of blocks, and tie bars are present in the lateral grooves separating blocks in other rows of blocks. Void areas in the shoulder area of the tire enhance traction in mud and sand.

BACKGROUND

The invention relates to pneumatic tire treads as well as to pneumaticradial tires comprising such a tread, such tires being suited for use onor off-road.

Off-road tires tend to have an aggressive, all block tread pattern,which provides good traction by digging into the ground. When used onpaved surfaces, however, the tires cause a rough ride and tend to bevery noisy. Partly because of the roughness of the ride, such tires havea high rolling resistance on paved surfaces.

It is known in the art that ribbed tires, i.e. tires having continuous"ribs" of tread rubber annularly around the tread, provide a smoothquiet ride on paved surfaces. Such tires provide poor performance whenused off-road since there are no aggressive blocks for digging into theground.

It is an object of this invention to provide a tire that provides goodoff-road traction, while providing a quiet, smooth ride on pavedsurfaces, as compared to conventional off-road tires.

It is a further object to provide a tire tread having an improvedrolling resistance, as compared to conventional tires used off-road.

SUMMARY OF THE INVENTION

The tire of the invention comprises at least a pair of annular beads, atleast one carcass ply wrapped around the beads, a ground contactingtread portion disposed over the carcass ply in a crown area of the tireand sidewalls disposed between the tread and the beads. The groundcontacting portion of the tread has five rows of blocks annularly aroundthe tire comprising two rows of shoulder blocks, a center row of blocksand two intermediate rows of blocks. The center row of blockscorresponds substantially with the equatorial plane (EP) of the tire andnarrow grooves between blocks in the center row have a width of 1% to 5%of the length of a block. Similar narrow grooves separate an least aportion of the shoulder blocks.

In the illustrated embodiment of the invention, narrow lateral groovesin the shoulder rows of blocks and in the center row of blocks areangled in substantially the same direction with respect to the EP of thetire, and conventional lateral grooves, between the blocks in theintermediate rows, are angled substantially in the opposite direction.The narrow grooves have an angle of 30°/150° to 60°/120° with respect tothe EP of the tire. The conventional grooves have an angle of -125°/-55°to -140°/-40° with respect to the EP of the tire.

Hook shaped sipes are located in all the blocks on the tire. Most of thehook shaped sipes are oriented substantially laterally in the tire. Someof such sipes in the shoulder blocks are oriented substantially parallelto the EP of the tire.

Tie bars are located in the grooves between the intermediate row ofblocks, and such tie bars have a depth of 60% to 80% of the nominaltread depth (which is the depth of a circumferentially extendinggroove).

A tire tread, substantially as described with respect to the tiredescribed above is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a tire embodying a tread made in accordancewith the invention;

FIG. 2 is a perspective cross section view of the tire shown in FIG. 1;and

FIG. 3 is an enlarged fragmentary view of a portion of the tread of thetire of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of this invention, a pneumatic radial tire is a tirewherein the cords of the carcass reinforcement which extend from bead tobead are substantially parallel to one another and are laid at a cordangle between 75° and 90° with respect to the equatorial plane (EP) ofthe tire. As used herein, an equatorial plane means a planeperpendicular to a tire's axis of rotation and passing through thecenter of its tread, midway between the sidewalls of the tire. The terms"radial" and "radially" are understood to refer to directions that areperpendicular to the axis of rotation of a tire, the terms "axial" and"axially" are used herein to refer to lines or directions that areparallel to the axis of rotation of a tire and the terms "lateral" and"laterally" are understood to refer to directions going from onesidewall of a tire towards the other sidewall of a tire. "Groove" meansan elongated void area in a tread that may extend circumferentially orlaterally in the tread in a straight, curved or zig-zag manner. Aconventional groove usually has a width which comprises about 15% toabout 30% of the length of an adjacent block. A "sipe" is a groovehaving a width in the range from about 0.2% to 0.8% of the tread width.Sipes are typically formed by steel blades, having a width of 0.4 to 1.6mm, inserted into a cast or machined mold; inasmuch as sipes are sonarrow, they are illustrated by single lines. "Tread width" (TW) isdefined as the greatest axial distance across a tread, when measured(using a footprint of a tire,) laterally from shoulder to shoulder edgeon the cavity shape, when mounted on the design rim and subjected to aspecified load and when inflated to a specified inflation pressure forsaid load. The tread arc width (TAW) describes the total curved treadextension from shoulder to shoulder edge. Obviously, as the tire treadis curved, TAW is always a few percent larger than TW. The footprintwidth is typically slightly smaller than TW, as the shoulder (which isusually round not sharp) is not fully in contact with the road. Axialwidths and other widths of other tread features or components aremeasured under the same condition as the tread width. By "footprint" ismeant the contact patch or area of contact of the tire tread with a flatsurface under normal load, pressure and speed conditions.

It is to be understood that the present invention relates to new tires,retreaded tires, and tire treads in strip form being at least partlyvulcanized and having a pattern of grooves and blocks integraltherewith.

With reference now to FIGS. 1 and 2, a pneumatic tire 10 having a groundcontacting tread 11 is illustrated. The treads are adjacent to shoulders12 which are in turn adjacent to sidewalls 13 that extend radiallyinwardly from the tread to the area of bead 14. Carcass plies 15 wraparound bead 14 and support the tread in a crown area of the tire.

Referring now to FIG. 3, there is represented an enlarged fragmentaryview of the tread of the tire shown in FIG. 1. The tread 11 has fourcircumferentially extending straight grooves 4-7 therein, spaced apartaxially across the surface of the tread and dividing the tread into fivecircumferentially extending rows 31-35 (hereafter respectively referredto as center row 31, intermediate rows 32, 33 and shoulder rows 34, 35)of elastomeric blocks 21-25. By circumferentially extending "straightgroove" is meant that the groove edges are substantially parallel to oneanother and the centerlines of circumferentially extending grooves aresubstantially straight. The circumferentially extending grooves 4-7 havea width ranging between 3% and 6% and preferably a width of 4% and 5% ofthe tread arc width. It is understood that there are twocircumferentially extending grooves on either side of the equatorialplane in the illustrated embodiment, but the number may vary inaccordance with the size of the tire.

As measured in a tire footprint, the axial widths RW1-RW5 of the centerblocks, each of the blocks of the intermediate row and the blocks of theshoulder rows, delimited by the axial position of the centerline of thebordering circumferentially extending straight grooves, are respectivelyfrom about 14% to 18%; 15% to 20% ; and 22% to 28% of the tread width TWof the ground engaging portion of the tread.

The blocks 21-25 of elastomeric material in each of the rows 31-35 aredisposed circumferentially about the tire in a repetitive manner and ina given row they have substantially the same geometric shape. Theinclination of the walls which are not parallel to the equatorial planeand the widths of the grooves separating the blocks in each row dependon the pitch variation, as is practiced in the industry for tire noisereduction.

The generation of noise while running on pavement is a particularproblem encountered in the use of tires designed for use off-road. Thetread of the invention minimizes the noise (and it has been found,lowers the rolling resistance) of the tire by providing a narrow groovebetween a portion of the blocks in the shoulder row and between theblocks in the center row, and by angling the grooves at a substantialangle with respect to the EP. The small gap between the blocks, andtheir angle of orientation, causes the row of blocks to behavesubstantially like a rib when contacting pavement. That is, because ofthe angled grooves, the leading and trailing edge of the blocks areloaded into and exit the footprint incrementally, avoiding noise causedby the block edge, as a whole, striking the pavement. For similarreasons, the hum caused by distortion of a block, and its restoration toits original shape, is minimized.

Because the grooves separating the intermediate row of blocks aresomewhat wider, the transition from one block to the next is not quiteas smooth, but the wider grooves provide a more aggressive block elementfor traction. To buttress the blocks in the intermediate rows, tie bars62 are provided in the grooves to reduce hum and to provide support tothe blocks when they dig into the ground.

A further reduction of noise is obtained by skewing the blocks so thatno two blocks enter the footprint at the same time. As can be seen fromFIG. 3, none of the blocks across the tire are axially aligned.

Specifically, the block elements 22, 23 of the intermediate rows are, ina front view of the tire, delimited laterally by the circumferentiallyextending grooves 4-7 and from the neighboring block elements in thesame row by laterally extending zig-zag grooves having three branches40, 41, 42. The first branch 40 and the third branch 42, opening intothe circumferentially extending grooves 5 and 6 on either side of theblock row, have a varying width. The widths of the first and thirdbranches increase from the width equal to the width of a second branchof the groove, up to a width of 4-9 mm when they intercept acircumferentially extending groove.

The inclination of the centerlines of the first and third branches isbetween 80° and 90° with respect to the equatorial plane. Theinclination of the centerlines of the second branch is between 35°/145°and 55°/125° with respect to the equatorial plane in the intermediaterows, and between -35°/-145° and -55°/-125° in the center row.

As recognized by those skilled in the art, the angles formed by theintersection of two lines add up to 180°, but are complementary. As usedherein, and looking in the direction of arrow A in FIG. 3, lines thatare in the second and fourth geometric quadrants, when intersected by aline parallel to the EP of the tire, will be designated as positive andthe angle and its complement will be provided (e.g. 125°/55°). Linesthat are in the first and third quadrants will be designated asnegative, and will also be provided in terms of the angle and itscomplement (e.g. -55°/-125°).

The second branch 41, connecting the first branch 40 and third branch42, have a substantially constant width over their length and have anegative angle.

The lateral grooves 51 separating the blocks 21 in a center row 31, havesubstantially the same shape having branches 52, 53, 54 as describedabove with respect to the lateral grooves between blocks 22, 23 inintermediate rows 32, 33, except that the angle of the second branch 53is positive. In second branch 53, the groove is "narrow" and is about1-2 mm wide.

It is understood that all groove widths are measured perpendicular tothe centerline of the groove.

In the illustrated embodiment, there are three rows of blocks having asecond branch with positive angles of orientation (see the discussion ofthe shoulder blocks below), and two rows of blocks with a second branchhaving negative angles of orientation, and the blocks with negativeangles are wider than the blocks with a positive angle of orientation,the tread pattern is in balance and does not cause a pull to one side orthe other of the direction of travel.

The narrow grooves 53, 44 in the center row and the shoulder rows havean angle of 30°-60°/150°-120°, preferably 35°-55°/145°-125° with respectto the EP of the tire. The wider grooves 41 in the intermediate rowshave an angle of 40°-55°/140°-125°, preferably 45°-50°/145°-130° withrespect to the EP of the tire. The tie bars in the intermediate rowshave a depth of 60% to 80% of the nominal tread depth.

As used herein, "narrow" grooves have a width of 1% to 5%, preferably 2%to 4% of the length of an adjacent block element as measured at itscenterline length parallel to the EP of the tire. In contrast, moreconventional lateral grooves, such as those in the intermediate rows ofblocks, have a width of 8% to 25%, preferably 12% to 20% of the lengthof its adjacent block element.

In the illustrated embodiment, the narrow grooves are made using 1 mmthick blades.

In the intermediate and center rows, the depth of the first and thirdbranches is substantially equal to the nominal tread depth. In theintermediate rows, the depth of the second branch of the lateral grooveis between 60% and 80% and in the center row, between 40% and 60% of thenominal tread depth.

The ratio of the length of a second branch to the length of a first orthird branch is between 1 and 4, a ratio of 3 being preferred.

In a preferred embodiment, the block elements 24, 25 of the shoulderrows 34, 35 each comprise two portions, an axially innermost mainportion 26 and an axially outermost shoulder portion 27. The mainportion 26 has substantially the same geometric shape as the blockelements 21 of the center row 31, with the difference that thirdbranches 43 nearest to the shoulder of the laterally extending groovesspacing the blocks 24, 25 have a substantially constant width of between3 and 8 mm and form with the second branch 44 an angle between 110° and150°. The width of the second branch 44 is about 1 mm and theinclination is opposed to the inclination of the second branch in theneighboring row 32, 33. The remaining side of an the outermost shoulderportion 27 is delimited by the respective tread edge, TE₁ or TE₂. In apreferred embodiment, for appearance reasons and also to provide extratraction when the tire is operated in mud or sand, the branch 43 nearestto the shoulder opens into a wide void area 60, at tread edge TE₁ orTE₂, that continues into the shoulder. The second branch 44 and theneighboring portion of the branch 43 nearest to the shoulder haverespectively depths of 40% to 60% and 60% to 80% of the nominal treaddepth. It is to be understood that providing the shoulder blocks 24, 25with an extra shoulder portion 27 confers a high lateral stiffness tothe tire tread, thereby improving handling.

In a preferred embodiment of the invention, the blocks of the center andintermediate rows 31-33 include hook shaped sipes 45, 46 originating ina circumferentially extending groove 4-7, about halfway between twolaterally extending grooves. The sipes extend substantiallyperpendicular to the equatorial plane EP. Generally a sipe can have adepth equal to or less than the nominal depth of the tread; in theillustrated embodiment, the sipes have a first portion nearest to thecircumferentially extending groove having a depth of 15% to 30% of thenominal tread depth, and second portion towards the center of the blockwith a depth of 70% to 90% of the nominal tread depth.

The shoulder portions 27 of the shoulder block elements 24, 25 compriseat least one, preferably two, hook shaped sipes 47, 48 orientedsubstantially parallel to the equatorial plane EP, originating in groove43, nearest to the shoulder. In a preferred embodiment, the sipes 47, 48have a first portion nearest to the groove 43, with a depth of 15% to30% of the nominal tread depth, and second portion having a depth of 40%to 90% of the nominal tread depth, towards the center of the blocks 24,25. The main portion 26 of shoulder blocks 24, 25 include one or twostraight or hook shaped sipes 49 having an orientation and depthscomparable to those of the sipes 45, 46 included in the intermediaterows 31-33 of blocks 21-23, and shorter sipes 50.

The circumferentially oriented sipes in the shoulder blocks (the area ofhighest ground pressure in the tires of the invention) are believed tocontribute to a quiet ride.

The invention is further illustrated with reference to the followingexample.

EXAMPLE

A steel belted radial carcass tire of size 255/65 R 16 was made with atread width of about 200 mm and five rows of blocks. The rows of blockswere molded to have axial widths RW1 and RW2 of about 36 and 29 mmrespectively. The pitch ratios were set at 17, 21 and 26 (mm) and thetread included 70 circumferentially spaced pitches.

The nominal tread depth is about 9 mm, and the widths of the groovesdepend on their axial position in the tread and the pitch variation. Thecircumferentially extending grooves have a width of about 8 mm.

Two carcass plies comprising 1100/2 dTex polyester reinforcement cordsand an uncured cord density of 29 ends per inch (EPI) were used in theconstruction. The belt structure comprised two single cut pliesreinforced by 2+2×0.25 mm steel cords, having a cord density of about 22EPI, forming angles of about 20° with the equatorial plane, the anglesof the cables in different plies extending in opposite directions. Thetire included 2 turns of an overlay ply reinforced with 940/2 dTex Nylonhaving a cord density of 30 EPI, and a substantially zero degreeinclination with respect to the equatorial plane.

The tread has a net to gross ranging between 65% and 75% and comprisesan elastomeric compound having a modulus ranging between 4 and 12 MPa, aShore A hardness ranging between 55 and 70, an elongation superior to400% and a tensile strength superior to 14 MPa.

While certain representative embodiments have been described for thepurpose of illustrating the invention, it will be apparent to thoseskilled in the art that various changes and modifications my be madetherein without departing from the spirit or scope of the invention. Theinvention has for instance been described with a design showing fiverows of blocks; it can be envisioned for smaller tire sizes, to choose athree row design, suppressing thereby the two intermediate rows ofblocks, or for larger tire sizes to choose a seven row design, addingtwo intermediate rows of blocks. The features in common in each of thesedesigns is that the angles of inclination of the lateral grooves (withrespect to the equatorial plane) in neighboring rows of blocks areopposed, and that the widths of the grooves in the center and shoulderrows of blocks are narrow, less than 2 mm, and preferably about 1 mm inthe illustrated embodiments.

What is claimed is:
 1. A pneumatic tire having at least a pair ofannular beads, at least one carcass ply wrapped around said beads, aground contacting tread portion disposed over said carcass plies in acrown area of said tire and sidewalls disposed between said tread andsaid beads, wherein said ground contacting tread portion comprises fiverows of blocks comprising two rows of shoulder blocks, a center row ofblocks and two intermediate rows of blocks wherein said center row ofsaid blocks corresponds substantially with the equatorial plane (EP) ofthe tire and wherein narrow grooves between blocks in said center rowhave a width of 1% to 5% of the length of said block, wherein narrowgrooves separate at least a portion of said blocks in said shoulder rowsof blocks, and wherein grooves between blocks in said intermediate rowof blocks have a width of 8% to 25% of the block length and contain tiebars.
 2. The pneumatic tire of claim 1 wherein all narrow grooves areangled in substantially the same direction with respect to theequatorial plane (EP) of the tire, and grooves having a width of 8% to25% of the block length are angled in substantially the oppositedirection.
 3. The pneumatic tire of claim 1 wherein grooves betweenblocks in said center row cross the equatorial plane (EP) of the tireand have an angle of 30°/150° to 60°/120° with respect to said EP. 4.The pneumatic tire of claim 1 wherein narrow grooves between shoulderblocks have an angle of 30°/150° to 60°/120° with respect to saidequatorial plane (EP).
 5. The pneumatic tire of claim 1 wherein groovesbetween blocks in said intermediate rows of blocks have a width of 8% to25% of the block length and comprise an angle of 125°/55° to 140°/40°with respect to the equatorial plane (EP) of the tire.
 6. The tire ofclaim 1 wherein hook-shaped sipes are located in all said blocks.
 7. Thetire of claim 6 wherein hook-shaped sipes in said shoulder blocks areoriented substantially parallel to said equatorial plane (EP).
 8. Thepneumatic tire of claim 1 wherein said tie bars have a depth of 60% to80% of the tread depth.
 9. The pneumatic tire of claim 1 wherein saidtie bars have a depth of 60% to 80% of the tread depth.
 10. A tire treadhaving five (5) rows of blocks comprising two (2) rows of shoulderblocks, a center row of blocks and two (2) intermediate rows of blockswherein said center row of said blocks corresponds substantially withthe center line of the tread and wherein narrow grooves between blocksin said center row have a width of 1% to 5% of the length of said block,and wherein narrow grooves separate at least a portion of said blocks insaid shoulder rows of blocks and wherein grooves between blocks in saidintermediate row of blocks have a width of 8% to 25% of the block lengthand contain tie bars.
 11. A tire tread of claim 10 wherein all narrowgrooves are angled in substantially the same direction with respect tothe center of the tread, and grooves having a width of 8% to 25% of theblock length are angled in substantially the opposite direction.
 12. Atire tread of claim 10 wherein grooves between blocks in said center rowcross the center line of the tread and have an angle of 30°/150° to60°/120° with respect to said center line.
 13. A tire tread of claim 10wherein narrow grooves between shoulder blocks have an angle of 30°/150°to 60°/120° with respect to said center line.
 14. A tire tread of claim10 wherein grooves between blocks in said intermediate rows of blockshave a width of 8% to 25% of the block length and comprise an angle of-(125°/55°) to -(140°/40°) with respect to the center line of the tread.15. A tire tread of claim 10 wherein hook-shaped sipes are located inall said blocks.
 16. A tire tread of claim 1 wherein hook-shaped sipesin said shoulder blocks are oriented substantially parallel to saidcenter line.